Please refer to FIG. 1, it is the schematic diagram which shows the current and voltage waveforms related to the reverse recovery of a diode in the prior art. At the moment when a diode is turned off, this diode does not have the reverse blocking capability due to the stored charges, thus a reverse current If is generated. When the diode has the reverse blocking capability at the moment t1, the absolute value of the current will drop dramatically so as to generate relatively high voltage spikes due to the inductance in the loop of the diode. In FIG. 1, ID1 is the conductive current of the diode, and VD1 is the voltage across the anode and cathode of the diode.
Please refer to FIG. 2, it shows the schematic circuit diagram of a typical full-wave rectifier circuit in the prior art. Assume that the waveform of the input voltage for rectifying is a square wave having the positive and negative wave-amplitudes. The operational procedures of the rectifying can be divided into: the rectifying stage, the continuation stage, and the recovery stage, and described as follows.
1. Rectifying stage: Please refer to FIG. 3. In which, the voltage V1 is positive, the diode D1 is conductive, the current flowing through the diode D1 is IL1, the diode D2 is turned off, and inductors Ls1 and Ls2 represent the leakage inductance of the transformer respectively.
2. Continuation stage: Please refer to FIG. 4. In which, the voltage V1 is zero, the diodes D1 and D2 are conductive, and the sum of the currents flowed through the diodes D1 and D2 is IL1 
3. Recovery stage: Please refer to FIG. 5. In which, the voltage V1 drops from zero to negative value, the diode D1 enters the reverse recovery stage having a current of If, and D2 is conductive having a current equals to the sum of the currents IL1 and If. When the diode D1 has been recovered till the moment t1, the diode D1 becomes to have the reverse blocking capability because of its' relatively high impedance value, and the ripple currents of the inductors Ls1 and Ls2 will generate the relatively high voltage spikes across the anode and the cathode of the diode D1. Due to the symmetric feature of the circuit, the diode D2 will have the reverse recovery problem too.
If the voltage spikes are not snubbed, the high-voltage diodes must be employed since the values of these voltage spikes could be several times of the rated reverse voltage of the diode. But, the forward voltage drops of the high-voltage diodes are higher than those of the low-voltage diodes in general. Thus, the power losses of the circuit would be increased dramatically due to the employment of the high-voltage diodes.
Please refer to FIG. 6, it is the schematic circuit diagram of the first clamping circuit for resolving the reverse recovery problem of the diode in the prior art. The operational principles of this circuit are: 1. When the diodes Ds1 and Ds2 enter the reverse recovery stage at the above-mentioned moment t1 and appear to have the relatively high impedances, the electrical energies of the leakage inductances are stored in the capacitor Cs1 or Cs2 through the diode Ds1 or Ds2 respectively; 2. The electrical energies stored in the capacitors Cs1 and Cs2 are sent to the load through the switches SA and SB. The advantages of this clamping circuit are: having a relatively good clamping effect and having no loss. The disadvantage of this clamping circuit is: the controlling of the circuit is relatively complex.
Please refer to FIG. 7, it is the schematic circuit diagram of the second clamping circuit for resolving the reverse recovery problem of the diode in the prior art. The operational principles of this circuit are: 1. When the diodes Ds1 and Ds2 enter the reverse recovery stage at the above-mentioned moment t1 and appear to have the relatively high impedances, the electrical energies of the leakage inductance are stored in the capacitor Cs1 or Cs2 through the diode Ds1 or Ds2; 2. The electrical energies stored in the capacitors Cs1 and Cs2 are sent to the primary side Np of the high-voltage transformer T2 through the switches Sx. The advantages of this clamping circuit are: having a relatively good clamping effect and having no loss. The disadvantages of this clamping circuit are: having the safety regulation problem regarding the high-voltage transformer T2 and having relatively lower efficiency.
Please refer to FIG. 8, it is the schematic circuit diagram of the third clamping circuit for resolving the reverse recovery problem of the diode in the prior art. The operational principles of this circuit are: When the diodes VD1 and VD2 enter the reverse recovery stage at the above-mentioned moment t1 and appear to have the relatively high impedances, the electrical energies of the leakage inductances are transferred to the primary side of the transformer through a forward converter. The advantage of this clamping circuit is: the controlling of the circuit is relatively simple. The disadvantages of this clamping circuit are: having a relatively worse clamping effect, having the safety regulation problem regarding the high-voltage transformer, and having relatively lower efficiency.
Keeping the drawbacks of the prior arts in mind, and employing experiments and research full-heartily and persistently, the lossless clamping circuit of the power converter having a relatively higher efficiency is finally conceived by the applicants.